Carboxylic acids are, by definition, Brønsted acids that can donate a proton to ther
species with lone pairs.
They are generally weak acids (vs. the hydronium ion), so the equilibrium above lies far to the left. For something like acetic acid, the pKa is 4.75, so K for the above expression is 10-4.75, which is about 0.00001. (You have probably considered in General Chemistry the problem of vinegar--a 5% to 8% by weight solution of acetic acid in water. This works out to be approximately 1 M; the expected, and observed, pH is 2.4.)
However, carboxylic acids are much more acidic than alcohols (pKa =
16-18). One explanation for this is the resonance delocalization afforded by the
carbonyl group:
Top: R=tert-Bu Bottom: R=tert-Bu Turn on ESP maps |
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Use the structures below to examine the impact of structure on C-O and C-C bond lengths.
| Turn on the distance monitor with the button below, then select a set of
bonds. Then click on any two atoms; the distance will be displayed on the status bar of
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(This is not the entire story; the electron withdrawing nature of the CO group
helps, compared to the electron donating nature of the R group. A hair-splitting
question is whether the resonance in the carboxylic acid is as important as the resonance
in the carboxylate.)
Go on to Acidity Page 2.